1. Field of the Invention
This invention pertains to the general field of testing of light-emitting materials. In particular, it pertains to an improvement in the apparatus used for characterizing the electroluminescent properties of semiconductor wafers.
2. Description of the Prior Art
The characterization of light-emitting semiconductor structures at the wafer-level (i.e., after forming the p-n junction and the active quantum well layers, but prior to the chip processing steps) is typically carried out with a non-destructive wafer probe. A conductive probe is temporarily placed in contact with the top of the epi-wafer (p-GaN) layer while an electrode contacts the n-GaN layer through either the edge of the wafer or through other means that allow access to the n-GaN layer. Such typical layout is illustrated in FIG. 1. When energized, the conductive probe, the semiconductor p-n junction structure on the wafer, and the electrode form a temporary light-emitting device. By injecting a known current into the junction, light will emit from the device and the spectral properties and their relationship with the electrical properties can be measured and characterized.
Although the method of using conductive probes for semiconductor wafer measurements and tests has been known in the field, the issues of making good, consistent probe-wafer contact with repetitive results are still problematic challenges that vary from application to application. For light-emitting wafer testing, a well-defined uniform contact area with minimal contact resistance is essential. Therefore, the probe material should be stable under a variety of electrical drive conditions.
One major challenge is the precise estimation and consistent repetitiveness of the contact area between the probe and the surface of the wafer, which affects conductivity and all related measurement parameters. U.S. Pat. No. 7,679,381 (issued to Ma) describes a probe that preferably includes a conductive deformable tip and a pressure control that together ensure a good contact with the wafer under test as various measurements are taken across its surface. However, it was found that these two components alone do not always ensure repeatable measurements.
As described below, an improved probe was obtained by using a shaped and polished spherical probe where the contact area with the flat surface of the wafer was controlled by estimation from an elastic contact model. However, a persistent problem remained in the fact that repeated measurements of the same wafer spot under controlled test conditions showed a consistent but inexplicable drift in the measured voltage and optical response resulting from the application of the same level of current. This phenomenon was noticed without any other change in any of the observable parameters available to the user. Because these probes are used primarily to test wafers at multiple locations, this problem raises serious doubts about the credibility of successive measurements taken at each location and throughout the wafer surface. The present invention describes a simple solution accidentally discovered during development work directed at controlling the probe conditions.